BK virus infection of the renal allograft has become a matter of considerable concern. We have played a leading role in defining the clinical course of these patients. Our effort in this area has been funded by an NIH sponsored RO-1 grant for the past 3 years. We have now prepared a competing continuation grant application to continue our study of this important problem. Specific Aim # 1: To characterize the sequence diversity and phylogenetic evolution of BKV, and determine its potential impact on molecular diagnostic assays in clinical practice. We hypothesize that variant BKV strains are compromising our ability to (a) determine the true prevalence of viral infection, and (b) accurately quantitate the viral genomic load in clinical samples obtained from individual patients. It will be our goal to clarify the rate at which new viral strains are developing in nature, and to determine whether PCR-based diagnostic assays will also need to be continuously adapted to compensate for this continuing evolution. Specific Aim # 2: To delineate host and viral factors that affect PV replication in the kidney. We propose to directly measure viral replication rates in mutant and wild type viruses in a cell culture system. In addition, we will determine if the risk of developing BK viruria and viremia is dependent on donor or host polymorphisms in genes capable of modulating host- pathogen interactions. Specific Aim #3: To examine the impact of smoldering BKV infections on the development of chronic allograft nephropathy. We hypothesize that asymptomatic viruria, which occurs in approximately 35% of our patients, represents a smoldering BKV infection that contributes to the development of chronic allograft nephropathy. To address this issue, we will use archival biopsy material to assess the serial progression of renal dysfunction in virus infected patients. Specific Aim # 4: To use 3- dimensional homology modeling and virtual screening technology to discover anti-BKV drugs capable of binding functional domains on the VP-1 and T-antigen molecules. Given the growing importance of BKV infection in renal transplant recipients, there is a pressing need to develop anti-BKV drugs suitable for clinical use. We hypothesize that anti-BKV drugs can be discovered by screening chemical libraries for compounds that can bind to two key viral proteins, namely, viral capsid protein-1 (VP-1) and large T antigen.